Plastic optical devices are generally superior to glass-based optical devices having the same constitution in that they are easy to produce and work and they are inexpensive. Recently, therefore, their applications have been tried to various fields of optical fibers, optical lenses and optical waveguides. Of such optical devices, plastic optical fibers (hereinafter referred to as POFs) are flexible, though having a disadvantage in that their transmission loss is relatively larger than that of glass-based optical fibers since their nude wires are entirely formed of plastics. In addition, they are lightweight and have good workability, and, as compared with glass-based optical fibers, they may be readily produced as large-diameter fibers. Still another advantage of POFs is that they may be produced at low costs. Accordingly, various investigations have been made for their applicability to short-distance optical communication transmission media not involving a problem of transmission loss.
In general, a plastic optical fiber comprises a core of an organic compound with a polymer as its matrix, and a clad (outer shell) of an organic compound having a different refractive index from that of the core (generally having a low refractive index). Recently in particular, a refractive index profile, plastic optical fiber, in which the core has a refractive index profile that varies from its center to its outside, has become specifically noticed as an optical fiber having a high transmission capacity, since it accepts a broad optical signal zone for transmission through it. One method proposed for producing the optical fiber of the type comprises preparing an optical fiber preform by the use of a refractive index-controlling agent, and then stretching the preform.
In preparing the preform, in general, a monomer for it is polymerized with a thiol such as n-dodecanethiol serving as a chain transfer agent added thereto. This is because if the polymerization is carried out in the absence of a chain transfer agent, then the molecular weight of the polymer produced may be too large and may be therefore difficult to stretch. However, it is known that some chain transfer agent may remain in the polymer produced and may increase the transmission loss through the polymer. In particular, this is remarkable when a fluorine-containing monomer is polymerized to give a polymer material for optical devices. To solve the problem, using a perfluoroalkyl group-containing thiol as a chain transfer agent is under investigation (JP-A 2003-192708). However, the chain transfer agent in this reference is limited in point of the type of monomer that may be combined with it, and its latitude in general use thereof is narrow.
On the other hand, for solving the problem of an odor of a thiol-based chain transfer agent, it is known that an ester structure-containing thiol may be effective in polymerization of a styrene-acrylic monomer (JP-A 2004-224708, 2004-224840). However, no one knows an example of using it for optical devices.